This is one I built, without any plan, just put it together to be a shooter. It is .56 cal, and 9" long. I've had it 7 years now, and yes it was proof tested, both provisional and definative proof loads were fired 3 times each, and each time the bore was air gauged, and the barrel wall was checked with ultra sonic equipment. (Note: four prior attempts failed one or more of these or a visual inspection due to poor material selection)
Service load of this one is 55 gr of 2F with 3/8 oz of #6 shot (usually just launches lubricated cotton balls with 70 gr of pyrodex RS.